Microporous membranes are prevalent in the chemical, food, pharmaceutical and medical industries where they are used to separate desired and undesired components of process streams, for example, to remove impurities by filtration or to separate and retain precious or useful particulate species. Microporous membranes are also used in custom apparel such as outerwear, where they provide breathability and yet protect the wearer from the elements such as wind and rain. They are also used in the fabrication of protective masks and apparel to help exclude toxic particulate species such as carcinogenic aerosols, spores and bacteria. In all of the aforementioned applications, performance is greatly limited by the largest pores in the membrane because the largest pore controls the size of the particulate that can be excluded and the majority of flow is relegated to the larger pores, such that the smaller pores may give a higher porosity number to the membrane but contribute little to overall flux. Hence, it is desirable to be able to produce porous membranes with little or no variation in pore size.
Uniform pores in planar films can be created by many different fabrication techniques. For example, uniform capillary pores can be created by ion bombardment and track-etched processes. They can also be created using laser ablation, ion beam etching or optical lithography. But all these micro-fabrication processes are limited by one or more of a variety of factors such as cost, a limited number of suitable material substrates, the inability to create large-area membranes, and low porosity.
Membranes without the open pores above are used to separate chemical species by permitting diffusion of some and not others. Life itself is sustained by selective diffusion through cellular lipid membranes, desalination is used worldwide to make fresh or potable water from sea or brackish water; likewise, gas purification, kidney dialysis and many other chemical separations are known as entropic driven processes. Many materials that have high selectivity that could be used as membranes are not used as the materials themselves have poor physical properties that make them impractical to use as a large area membrane of commercial value.
Membranes and sheet structures can also be created from a “fiber-on-end” (FOE) process wherein multicomponent fibers with microfeatures are assembled in a preferred direction and then consolidated or sintered together to create a defect-free structure. When this solid structure is cut or sectioned in a direction that is perpendicular to the orientation of the fibers, membranes and sheets with microfeatures are created. Fiber-on-end arrangements have been found to have useful properties for membranes and capillary arrays. Hand lay-up of such materials is possible, but not practical for commercial manufacturing.
One method of making the fiber-on-end materials is to arrange pre-cut thermoplastic fiber lengths into a cavity of a press die. The die is closed and heat and pressure are applied, so that the walls of the fibers soften and fuse together. The amount of pressure and heat applied will depend on the composition and structure of the fibers. If too much pressure is applied, hollow fibers could collapse or the cores of sheath-core fibers could be distorted. If insufficient pressure is applied, the fibers may fuse only partially, leaving behind voids and defects. It is also desired to apply enough pressure to allow the fibers near the center to be compressed, yet avoid crushing fibers near the outside. Heat is also applied externally and transfers through the mass of fibers to the reach the center. Careful application of heat and a sufficient rate of heat transfer can allow one to avoid degrading, distorting or melting the cores of the outermost fibers while still allowing the fibers located near the center to fuse.
Similar care is taken when making fiber-on-end materials using binders or solvents. Sufficient time is needed for the binder or solvents to diffuse into the surface and if appropriate evaporate. If a heat-activated binder is used, the rate of heat transfer can be limiting, and, care is taken to ensure that the inner most fibers before the outer fibers are cured.
It can be seen that making fiber-on-end materials with large dimensions by this method is limited by heat transfer rates and would likely require careful control and choice of time and temperature.
In European Patent Applications 195860A1 and 167094A1, parallel fibers are consolidated by winding the fibers on a drum and then bonding or thermally fusing them into a solid that is later skived in a direction perpendicular to the parallel fibers. The fibers, having been arranged concentric to the surface of the winding drum, must be sliced in a radial direction with respect to their winding orientation. This is accomplished by cutting off the consolidated fiber layer, pressing it flat, cutting sections of the flattened layer, reorienting the sections by ninety degrees, fusing the sections together into a block, cutting the blocks again into trapezoids, arranging the trapezoids around the periphery of a support drum and skiving a layer, perpendicular to the fiber axis, to form a membrane. In EP0167094, a solid cylinder of sea polymer is made at a temperature above the sea melting point, then cut axially into four segments which are pressed flat prior to making thin cuts into this flattened segment. This pressing flat of a thick fused polymer block, which is reinforced with small polymer cores, places high extensional stress on those cores on the smaller inside curvature of the quartered section and high compressional stress on cores nearer the outside larger curvature. This could impose high distortion to the cores and give non-uniform capillary structures. The method in EP195860A1 and EP167094A1 requires multiple handling steps and is not readily adaptable for large-scale, continuous or potentially automated operation. Heat transfer rates also limit how quickly each fusing step can be accomplished with thermoplastic or reactive bonding agents. These features limit the productivity of these methods and practical membrane size.
There thus remains a need for a process capable of making fiber-on-end materials of large planar dimensions, e.g., one meter wide or more, in an at least partly continuous or automated manner.